Chosen Solution

The oldest Firefox that supports TLS 1.2 is Firefox version 23,
but I'm definately not going to suggest to use an older Firefox version .....

NOT recommended, but you can set any Firefox version to use TLS 1.2 :

Type in the address bar about:config and press Enter
(promise to be careful, is asked)

Type in the search bar security.tls.version.max

and set its value to 3

Then close and restart Firefox.

Hello Pat07,
The oldest Firefox that supports TLS 1.2 is Firefox version 23,
but I'm definately not going to suggest to use an older Firefox version .....
NOT recommended, but you can set any Firefox version to <BR>use TLS 1.2 :
Type in the address bar '''about:config''' and press Enter
(promise to be careful, is asked)
Type in the search bar '''security.tls.version.max'''
and set its value to '''3'''
Then close and restart Firefox.

You shouldn't set security.tls.version.max to a value higher than its current default.
If a higher TLS version isn't enabled in Firefox then this is done for a reason (stability or unfinished support).
This happened with TLS 1.3 for a long time (52-59) that got only enabled in Firefox 59 as a test and by default in 60and later.

Actually this enabling TLS 1.2 (and TLS 1.1) happened in Firefox 27 (TLS 1.1 was skipped).
Bug 861266 - Implement TLS 1.2 (RFC 5246) in Gecko (Firefox, Thunderbird), on by default [27]
You shouldn't set security.tls.version.max to a value higher than its current default.
If a higher TLS version isn't enabled in Firefox then this is done for a reason (stability or unfinished support).
This happened with TLS 1.3 for a long time (52-59) that got only enabled in Firefox 59 as a test and by default in 60and later.